xlvi FACTS AND OBSERVATIONS. 



amounts. The recurrents are treated in like manner, but the number of horizons which they 

 occupy is also shown. Thus, of recurrent Trilobites in the lower stage sixty-nine are seen in two 

 horizons, fifteen are in three, and two in five ; while the middle and upper stages are similarly 

 treated. Recurrent Trilobites are 14 per cent, of the whole order. In like manner Table X gives 

 the percentage of recurrent species throughout the entire Silurian fauna. 



We have not yet learnt always to distinguish a recurrent from a typical species ; but this 

 may sometimes be done from its retaining the peculiarities of the native stage, and from the marks 

 of migration it may carry. The individual we happen to take in our hands may not have changed 

 its horizon, being the offspring of old residents, partaking, however, of the epigenic alterations 

 passed through by the deposit holding it *. 



The vegetation of the Silurian epoch enjoys some vertical range, but chiefly affecting its genera. 

 Table X shows that out of fifty-nine species (known in 1866), five pass into other stages, but only 

 into such as are coterminous. Generically plants enter many horizons. Of the genus Palceophycus, 

 ten species are in the Primordial of Labrador, New York, &c. ; others arrive at the middle stage ; 

 and one has been discovered in the Upper Silurian of the Baltic sea. 



Among the Annelida, Buthotrephis has four Primordial forms, most of the others being Middle- 

 Silurian, while B. succulens, according to Prof. Geinitz, is in the Primordial at Lobenstein (Reuss, 

 Germany), and in the Trenton limestone of New York. 



The three species of Rusophycus occupy (each separately) the Chazy beds, the Clinton group, 

 and the Eurypterus-limestone of North-east America, three horizons of very different dates. 



The conditions favouring recurrence, or rendering it possible, are simplicity of structure, 

 fecundity in reproduction, longevity, the power of locomotion, facility of transportation, and con- 

 ditions continuous, or nearly so. While sediment is slowly accumulating, generations mount up 

 with the increasing thickness, until they often find themselves among strange life, and they them- 

 selves are called recurrents. All this is greatly aided by a steady medium like the sea, and an 

 occasional failing in power on the part of opposing circumstances. 



Recurrency in marine life, ancient or modern, is universal, and is common to all forms of 

 organic existence, and to every part of time, the act growing in frequency through every succeeding 

 epoch up to the present day. 



Mollusks may have recurred in companies, as they must often travel in groups ; but instances 

 are unknown to me, nor are they easy of detection. The fact which we are discussing shows that a 

 marine creature is not necessarily confined to any one community, but that both it and its young 

 may find good homes in several successively. 



Recurrence is a measure of viability, that is, of capacity for enduring change of food, pressure, 

 temperature, &c. ; and the number of recurrents becomes a measure of new conditions, the more 

 numerous the recurrents the less being the change. 



By far the greater number of derived fossils congregate about the first layers of new strata! 

 subdivisions, and then are replaced by the Autochthones of Agassiz, the native mollusks. This is well 

 exemplified in the Trilobites of Bohemia. Pennsylvania and New York exhibit similar facts, 

 together with a remarkably great intermingling of fossils in the contiguous beds of two stages. In 

 Tennessee, the Niagara and Lower- Helderberg groups, so widely apart in New York, are inseparable 

 in their molluscan life and mineral condition for the thickness of thirty feet f ; but under these 

 circumstances we cannot be said to be dealing with full and true recurrency. 



* It is well to give a summary of these alterations (rnetamorphisms). Many rocks are apparently barren which 

 certainly once contained extinct life. They and their contents have been more or less transformed, and the latter even 

 obliterated. The rock has become hardened by assumed cleavages or crystalline forms. It may now be vesicular, have 

 received magnesia, lime, iron, sulphur, &c. by way of addition to its original composition ; and it may have developed 

 new minerals. 



t Bull. Soc. Geol. de France, xviii. ; Canad. Journ. i. 220, ii. 138 ; Geol. Report Tennessee, Prof. Safford ; Geol. 

 Report Pennsylvania, H. D. Rogers j Geol. Report, Logan, 1857, pp. 152, 156. 



